The present invention relates in general to an electronic circuit for operating arc discharge lamps, and more specifically to providing high frequency energy for starting an arc in a lamp and providing low frequency energy for running a lamp once an arc has been established.
Gas discharge lamps can be operated on either alternating current (AC) or direct current (DC). DC operation is often desirable because of the low cost of DC ballasts and because of the reduced amount of electromagnetic interference (EMI) associated with DC operation. However, some lamps, e.g. fluorescent lamps and high pressure metal halide lamps, exhibit undesirable characteristics when operated on DC. In these lamps, DC operation causes the active gas to be transported to the cathode region by the process of cataphoresis. This process causes decreased light output in fluorescent lamps and color shifts in high pressure metal halide lamps which use multiple active gases.
Cataphoresis has been avoided in DC operated high pressure metal halide lamps by positioning the lamp vertically with the lamp cathode on the top. However, any movement from this orientation changes the color of the light produced considerably.
Accordingly, it is desirable to use AC power to operate those lamps which would be subject to cataphoresis when operated on DC power. One well known technique for providing AC lamp power utilizes an AC to AC converter for periodically reversing the polarity of the lamp voltage. In lamp ballasts employing this technique, the lamp is operated at a high frequency, e.g. a frequency higher than the 60 hertz line frequency, in order to reduce the size of inductive and capacitive ballasting elements. High frequency operation also permits the use of resonant circuits for providing the high voltage levels needed to initially start the arc discharge.
Gas discharge lamps have also been operated directly from the AC power line. In these instances, it is also known to employ high frequency voltages for starting an arc and for helping to re-establish an arc in each new half-cycle in case the re-ignition voltage exceeds the available line voltage.
The lamp operating circuits using fast switching transitions are subject to undesirable amounts of EMI. As described in U.S. patent application Ser. No. 748,076, filed 6-24-85, of common assignment and filed concurrently herewith, a gas discharge lamp may be operated by an AC to AC converter synchronously with the AC power line for minimizing EMI conduction to the power line.
In an AC to AC converter for operating a discharge lamp, such as is shown in application Ser. No. 748,076, filed 6-24-85, it is desirable to use a bridge configuration of semiconductor switches in the converter which have as low a voltage rating as possible in order to reduce circuit costs. The maximum DC voltage supplied to the converter bridge from the rectifier connected to a utility's AC power line is approximately 180 volts. This voltage is typically sufficient to maintain the arc discharge once the lamp is in a normal running condition. However, voltages in excess of 1000 volts may be required to start the lamp.
Accordingly, it is a principal object of the present invention to provide a circuit for starting and operating gas discharge lamps in any orientation without the occurrence of cataphoresis.
It is a further object of the invention to start a gas discharge lamp with a high frequency AC signal and to operate the lamp with a low frequency AC signal under normal conditions.
It is another object of the invention to apply high breakdown and glow-to-arc transition voltages across a lamp connected to an AC to AC converter without the high voltages appearing across the semiconductor bridge switches.
It is yet another object of the invention to provide a lamp operating circuit whose functions coincide with the requirements of the lamp as it shifts between operating modes.
It is another object of the invention to provide a lamp operating circuit which results in very little conducted EMI on the power supply lines.